Documentation/features-refresh.sh: Only sed the beginning "arch" of ARCH_DIR

It should only sed the beginning "arch" of ARCH_DIR in features-refresh.sh,
otherwise loongarch is recognized as loong, that is not what we want.

Fixes: be99f610 ("Documentation/features: Add script that refreshes the arch support status files in place")
Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>
Link: https://lore.kernel.org/r/1670156327-9631-2-git-send-email-yangtiezhu@loongson.cn


Signed-off-by: Jonathan Corbet <corbet@lwn.net>